Electronic temperature control



Dec. 7, 1948. MCLENNAN 2,455,379

ELECTRONIC TEMPERATURE CONTROL Filed NOV. 14; 1945 F/Al/TE TEMPERATURE (flfFF/C/IFNT INVENTOR.

MILES A. M- ZE/wvmv Patented Dec. 7, 1948 UNITED STATES. PATENT OFFICE ELECTRONIC TEMPERATURE CONTROL Miles A. McLennan, Dayton, Ohio Application November 14, 1945, Serial No. 628,515

11 Claims.

The present invention relates to improvements in electronic temperature control devices and its principal object is to provide an automatic temperature control to be regulated by the heater element itself through change in electrical resistance of the heater element upon its change in temperature. A further object of the invention is a time cycling circuit wherein a condenser-resistor discharge circuit combination determines the length of time that the heater element is not supplied with current.

It is further proposed to balance a voltage derived from the flowing current through the heater element against another voltage derived directly from the supply source to control the temperature of the heater element.

Further objects and advantages of my invention will appear as the specification proceeds.

A preferred form of my invention is illustrated schematically in the accompanying drawing.

While I have shown only the preferred form of my invention, I wish to have it understood that various changes in modifications may be made within the scope of the claims hereto attached without departing from the spirit of the invention Referring to the drawing in detail, electrical conductors 2 and 3 are connected to an altemating current supply source through plug [0. A switch l2 may be used to open and close the circuit. Across the conductors 2 and 3, a filament transformer l4, having a primary [4a and secondary. l4b, is connected by means of conductors 40 and 4|. The secondary l4b supplies current through conductors I40 and N11 to filament l6 of a triode tube 20 and filament I8 of a rectifying tube 36. A relay 2!; having a coil 26a and an arm 26b is provided for controlling the voltage on the cathode 52 of tube 35. The coil 25a is connected to conductor 3 and to the plate 24 of tube 20. The arm 25b is arranged such that when the coil 26a is energized, conductors 3 and 45 are connected as may be seen from the drawing.

The cathode 22 of the tube 20 is connected with conductor 2 by means of a wire 8.

The transformer 28 consists of a primary 28a and secondary 28b. The primary 28a. is connected to conductor 2 by conductor 42 at one end and at the other end by conductor 43 to conductor 5, which in turn is connected to conductors 44 and 45. Conductor 45 is connected to the arm 25b of the relay 26.

One conductor of the secondary 28b of transformer 28 is connected to the primary 32a of a transformer 32 through conductor 4. The other conductor of the secondary 28b is connected to one end of the heater element 30 whose resistance increases with increase of temperature. The other end of the heater element is connected to the other end of the primary 32a by conductor 47.

The secondary 32b of transformer 32 is connected to the grid 34 of tube 20 by conductor 1.

Conductor 48, from the secondary 32b, connects the slider38b of the potentiometer 38. The re sistor 38a of the potentiometer 38 is connected by conductors 49 and 50 to conductors 8, 2 and 9.

A condenser 40 is connected to conductor 9 and to conductors 2, 49 and 8 by conductor 5i.

Conductor 9 is connected to the plate 53 of a rectifier tube 36. The cathode 52, of the tube 36 is connected with conductor 44. In one embodiment of my present invention as a blanket-heater temperature control, I use an indirectly heated cathode-type thyratron tube for the tube 23; a rectifier tube number 6X5 for the tube 36; a 2000 to 4000 ohm relay 26, with contacts capable of handling 2 amperes of current at 115 volts; a potentiometer 38 with 5 megohms of resistance; a fixed condenser 40, of l microfarad capacity: a transformer 32 capable of providing about 115 volts secondary at 1 to 2 volts primary.

The operation of my electronic temperature control device is as follows:

Alternating current from the source through plug [0 is supplied to conductors 2 and 3 which in turn supplies current through conductors 40 and 4! to the primary of the filament transformer l4. Induced current in the secondary I422 of the transformer I4 is used to heat the filaments 16 of the tube 20 and I8 of tube 36.

Alternating current from conductor 2 is supplied in one alternation to the cathode 22 of tube 20 through line 8. Assuming that the current direction is such that tube 2|! conducts, current then flows through the tube 20 from the cathode 22 to the plate 24 and through the coil 260. which in turn will move arm 26b to closed position connecting conductors 3 and 45. The magnetic action of relay 26 on arm 26b is a pulsating one due to the unilateral conduction characteristic of tube 20 when supplied with alternating current. However, by proper design of the relay, the relay contact will remain continuously closed as long as the average current conducted by tube 20 is sufficiently high. Current then will flow from the source through plug l0 into conductor 3, through the arm 26b to conductor 45, into the conductor 44, conductor 6, conductor 43, and the primary 28a of transformer 28. From the primary 28a, current will flow back to plug I 0 through conductors Q2 and 2, and the input source, during one alternation and reversing the path with the next alternation. Current flowing through the above described circuit will energize the primary 28a of the transformer 28.

Current is thus induced in the secondary 28b of transformer 28. The current is carried by conductor 46 through the heater element 30, through conductor ll. through the primary 32a of the transformer through conductor 4, and thence to the other end of the secondary 28b, reversing the path with the next alternation through the same circuit.

When the circuit first begins operation, the heater 30 is cool and its resistance is low compared with its resistance when heated. Consequently, the amount of current through the primary 32a is comparatively high, and the amount of voltage in the secondary 32b of the transformer 32 is relatively high. As one end of the grid 34 of the tube 20 is connected to the secondary 32b through conductor '1, the amount of charge carried by the grid 34 is proportional to the voltage in the primary 32a. This potential Will be high as long as the resistance of the heater element is low.

When the relay 26 is closed, as it is when the circuit is first turned on by the switch I2 as above described, alternating current will flow from the source, through the plug l0, through the conductor 3, relay 26, conductor 45, conductor 14 to the cathode 52 of the tube 30. Unidirectional current will flow from plate 53 through conductor 9 to the capacitor 40 and through the variable resistance 38, secondary 32b, conductor l, onto the grid 34. It will be readily seen that the grid 34 will be alternately negatively and positively charged due to the alternations of current in the primary 32a. Also, the grid 34 will have a negative charge imposed on it by virtue of the rectifying action of tube 30 and the capacitor 40-resistor 38 circuit. As the grid becomes more negative, the tube 20 has greater difficulty in passing current. When charge on the grid reaches a certain Value, the tube 20 will refuse to pass current altogether.

The negative charge impressed on the grid 34 by the tube 36 and capacitor-resistor circuit as aforesaid, will be added to, in one alternation. In the next alternation, a positive charge will be impressed on the grid in opposition to the negative impressed charge, and the charge on the grid may even become positive. If the effect of the positive charge reduces the negative charge sufliciently, tube 20 will conduct current. If the positive charge is not of sufficient strength, the tube 20 will not pass current and the relay will fall out. The amount of positive charge which will be impressed on the grid during alternation will become less and less as the resistance of the heater element 30 increases, and the voltage in the primary 32a decreases.

As the heater element 30 becomes hot, its re sistance will increase, and the current in the primary 522a will become less. The voltage induced in the secondary 32b will be correspondingly less, and will finally be of such value that the sum of the induced voltage and the unidirectional voltage will not allow current to flow through the tube. The relay 26, will then fall out as described above.

The capacitor 40, variable resistance 38 circuit will maintain a potential on the grid for a period of from 15 to 60 seconds, depending on the values used, which will prevent current from passing through the tube 20, and allow the heater element 3!! to cool. In the period from 15 to 60 seconds, the charge will leak off the grid 34 and the tube 20 will then begin to pass current again, energizing relay 26 and restoring the circuit to the on condition. If the heater element is then cool enough to permit the voltage in 32a to rise sufficiently to counterbalance the direct current voltage maintained on the grid by the tube 36 and the condenser-resistor circuit as aforesaid, the relay will remain energized. If not, the relay will open again and the 01f time cycle will repeat itself.

The position of the potentiometer arm 3% along the resistance 38a determines the temperature of the heating element at which the relay opens the circuit thus preventing a further increase in temperature.

While I have mentioned my electronic temperature control in reference to blanket-heater temperature controls, I wish to have it clearly understood that my electronic temperature control may be used wherever it is desired to control the temperature of any heater element Whose resistance varies with temperature. Examples of this use may be found in the control of electrical heating furnaces such as used in laboratories or industrial processes.

I claim:

. 1. In combination, an electrical heater element, having a source of alternating current, a supply circuit including an electrical valve and relay to direct said alternating current into said heater element, a control circuit for controlling the charge on the grid of said electrical valve through change in temperature of said heater element, a bias generator comprising an electrical valve associated with said grid to impress a potential on said grid while said relay is closed, and a capacitor-resistor circuit to maintain a charge on the grid of the aforesaid electrical valve for a predetermined period when said relay is open.

2. In combination, an electrical heater element having a source of alternating current, a supply circuit including an electrical valve and relay to direct said alternating current into said heater element, a control circuit for controlling the charge on the grid of said electrical valve through change in temperature of said heater element, a rectifier associated with said grid to impress a potential on said grid while said relay is closed, and an electrical time cycling circuit associated with said grid to maintain a charge on said grid of the aforesaid electrical valve for a predetermined period when said relay is open.

3. In combination, an electrical heater element having a source of alternating current, a relay with a relay coil and electrical valve in series connection, a transformer whose primary is energized when said relay is closed and whose secondary is in series connect-ion with the aforesaid heater element, a primary of another transformer in series connection with said heater element, whose secondary is connected to the grid of the aforesaid electrical valve and to a potentiometer, a fixed condenser in shunt with said potentiometer, and an electrical valve connected to said potentiometer and the primary of said first named transformer.

4. In combination, an electrical heater element having a source of alternating current, a relay with a 2000-4000 ohm resistance relay coil and electrical valve of the :triode type in series connection, a step-down transformer capable of supplying 8 to 10 amperes of current at 25 volts poafictdiac 5. tential, whose primary is energized when said relay is closed, and whose secondary is in series connection with the aforesaid heater element, a primary of another transformer of the current type providing approximately 115 volts potential secondary at l to 2 volts potential primary in series connection with said heater element, whose secondary is connected to the grid of the aforesaid electrical valve and to a 5 megohm variable resistor, a fixed condenser of 1 microfarad capacity in shunt with said variable resistor, and an electrical valve connected to said variable resistor and the primary of said first named transformer.

5. A controller for regulating the temperature of an electrical heating element comprising, a heating element the resistance of which is a function of its temperature, an electrical circuit supplying electrical current to said heating element, a switch in said circuit having open and closed positions preventing or permitting, respectively, current fiow to said element, a voltage generator coupled to said circuit and producing therefrom a voltage which is a function of the current in said circuit and through said heating element and therefore of the temperature of said element, a voltage supply circuit coupled to said electrical circuit producing therefrom a voltage which is a function of the voltage across said circuit and therefore of the supply voltage, a coupler combining said voltages in opposition, a comparison circuit responsive to a predetermined difierence in said voltages for actuating said switch to the open position, and a. delay mechanism subsequently actuating said switch to the closed posithan after a predetermined and fixed interval of time independent of the temperature or said element.

' 6. A controller for regulating the temperature of an electrical heating element comprising, in combination, a heating element the resistance of which is a function of temperature, an electrical circuit for supplying electrical current to said heating element, a switch in said circuit having open and closed positions for preventing or permitting, respectively, current flow to said element, a switch control responsive to resistance changes of said element for actuating said switch to said open position when the element reaches a predetermined temperature, a manually adjustable element associated with said switch control to ad- Just the resistance value at which said switch control actuates said switch to the open position, a control circuit actuated by said switch for rendering said switch control unresponsive to said resistance change when said switch is operated thereby, and a time delay control for holding said switch in the open position for a predetermined interval of time after said switch is actuated by said switch control, and actuating said switch to the closed position at the expiration of said interval.

'7. In a heating system having a source of electrical potential and a heating element the resistance of which varies with temperature, a controller comprising, in combination, an electrical circuit including an electrically operated switch for directing electrical energy from said source to said heating element, an electrical valve having a control electrode and connected to said electrically operated switch to cause said switch to open or close in response to the current through said valve, a bias circuit providing a manually variable bias potential to said control electrode, a control circuit applying a control potential responsive to changes in temperature of said heating element't'c said control electrode to oppose the effect of said bias potential and cause said switch to interrupt the flow of current to said heating element whenever said heating element has attained a predetermined temperature, and a delay control causing said electrically operated switch to close said electrical circuit a predetermined'interval of time following the opening-of said electrical circuit, whereby said heating element is maintained at a constant temperature dependent upon the adjustment of said manually variable bias potential.

8. For use in connection with an electrical heating element whose resistance changes with temperature, a controller comprising, in combination, a line connectible with a source of alternating current, a supply circuit including an electrical valve having at least one control electrode and'a relay to direct said alternating current into said heater element, a control circuit responsive to resistance change of said heater element caused by temperature change thereof to vary the potential of a control electrode oi. said electrical valve to cut off said valve as said resistance (caused by temperature increase) rises above a predetermined value, and to permit current iiow through said valve as said resistance (caused by temperature decrease) drops below said predetermined value, a bias circuit for impressing a manually variable bias potential on a control electrode of said valve while said relay is closed and which potential normally tends to cut off said electrical valve, said bias circuit serving to regulate said predetermined value of resistance of said heater element required to cut all said valve, and a holding circuit to maintain said valve in cut oil condition for a predetermined length of time after said cutting off by said resistance increase of said heating element.

9. For use'in connection with an electrical heating element adapted to derive energy from a source of current, a controller comprising, in combination,-an electrical valve having a control electrode and a relay controlled by said valve to govern the energization of said heater element in accordance with the flow of current through said valve, a conductor connecting said valve to said source of current, a voltage supplier coupled to said control electrode for producing a manually variable bias voltage, a voltage producing circuit responsive to the change in resistance of said heating element for producing a control potential, a coupler for applying said control potential to said control electrode of said valve in opposition to the effect of said bias voltage to cause said relay to open as the temperature of said resistance reaches a predetermined value dependent upon the magnitude of said bias voltage, a delay circuit maintaining said relay open for a predetermined interval of time after said element reaches said predetermined temperature and causing said relay to close at the end of said predetermined interval.

10. For use in a heating system having a source of electrical current and a heating element the resistance of which varies with temperature, a controller comprising, in combination, an electrical circuit for supplying electrical energy from said source to said heating element, an electrically operated switch for opening and closing said circuit, an electrical valve for passing current to operate said switch, said valve having control electrodes, a bias supply circuit applying a manually variable bias potential to at least'one of said control electrodes, a voltage producing circuit applyingv a control potential (variable in response to current flow through said heating element and so to change of ternperature of said heating. element) to at least one of said control electrodes in opposition to said bias potential, to regulate the passage of current bysaid electrical valve in accordance with the relationship between the value of said control potential and the value of said bias potential, whereby said switch is normally held closed when the temperature of said heating element is below a predetermined value determined by said relationship and is opened when the temperature is above the predetermined value, and a cyclic controller for, holding said electrically operated switch open for a predetermined interval of time followingv the openin of said electrical circuit and for closing, said switch at the expiration of said interval, whereby the amount ofhcurrent supplied to said heating elementv is regulated and said heating, element is maintained at a substantially constant temperature dependent upon the adjustment, of said manually variable bias potential.

11., For usein a heatingsystem having a source of alternating electrical current and a heating. element the resistance of which varies with temperature, a controller comprising, in combination,;an electrical circuit for conducting electrical energy from saidsource to said heating element,

a relay associated with said circuit and having an. actuating coil and contacts operated thereby to. open and close said electrical-circuit, a gaseous discharge device having a control electrode and connected in series with said actuating coil and said source of alternating potential to control theu'energization of said actuating coil in ac-' cordance with the flow of current through said gaseous discharge device,,a bias source including a rectifier and an adjustable resistance for pro-- ducing a manually variable bias potential, a bias conductor connecting said bias potential to said control electrode of said gaseous discharge device, a step-up 1 transformer having a primary circuitconnectedtin series with said heating ele- 8 ment and a secondary circuit. connected to said control electrode to produce an alternating control potential variable in response tothe flow of current through said heating. element and so to the temperature of said heating element, said. control potential serving to regulate the flow of current through said gaseous discharge device in accordance with the relative magnitudes of said bias and control potentials, the current flow through said discharge device serving to operate said: contacts to open said electrical circuit whensaid heating element reaches a predetermined temperature dependent upon the manual adjustment of said bias potential, and a cyclic controller for holding said contacts open for a predetermined interval of time each time said contacts are opened and closing said contacts at thev expiration of said predetermined interval.

MILES A. McLE-NNAN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS FOREIGN PATENTS Country Date Great Britain Nov. 18, 1930 Germany Oct. 21, 1936 Number Number 

